High speed image sensors have been widely used in many applications in different fields including the automotive field, the machine vision field, and the field of professional video photography. The technology used to manufacture image sensors, and in particular, complementary-metal-oxide-semiconductor (CMOS) image sensors, has continued to advance at great pace. For example, the demand of higher frame rates and lower power consumption has encouraged the further miniaturization and integration of these image sensors.
In addition to the frame rate and power consumption demands, image sensors are also subjected to performance demands. The quality and accuracy of the pixel readouts cannot be compromised to accommodate the increase in frame rate or power consumption.
For example, current image sensors with column analog-to digital conversion (ADC) circuits read out multiple pixels (e.g., a row of pixels) at the same time. When a large number of these read out pixels receive similar brightness, the pixels may cause shift in the apparent comparator output values of the rest of the pixels. This shift appears as a horizontal smear or horizontal streak noise on the image readout which is referred to as Horizontal Banding (or H-banding). H-banding is one of the major performance issues in image sensors. H-banding is caused by the accumulated disturbance to environment signals (references, power/ground nets, etc.) from the readout of a subset of pixels, which then affect the accuracy of reading out other pixels that also share these signals.
Darker signals usually toggle earlier depending on the ADC design and brighter signals that toggle later can be affected. Comparators coupled to pixels of the similar brightness would toggle around the same time. The effect of a plurality of comparators toggling around the same time would have an accumulated effect on the other comparators, resulting in H-banding.
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